1. Field of the Invention
The invention relates to a voltage adjusting circuit, and more particularly to a voltage adjusting circuit using built-in self test (BIST) technology.
2. Description of the Related Art
Integrated circuit (IC) devices usually include several internal voltage sources generating voltages with different values for operating the circuit. However, since process variation may cause some drifting of the output voltage of the voltage sources, voltage sources in the IC devices need to be adjusted by a trimming circuit after manufacturing of the IC devices. The trimming circuit repeatedly tests the voltage sources so as to adjust their output voltage to approach a target value.
FIG. 1 shows a conventional trimming circuit. An integrated circuit (IC) 100 comprises voltage sources V1-V4, pads Pad 1-Pad 4 and input/output pad I/O Pad. The output voltages generated by the voltage sources V1-V4 are respectively output via the pads Pad 1-Pad 4. The testing machine 15 is an IC testing machine disposed outside of the IC 100, which receives the output voltages supplied by the voltage sources V1-V4 via the pads and compares the output voltages with target voltages that are set inside of the testing machine 15. The comparison results are fed back to the voltage sources V1-V4 via the I/O Pad to further adjust the output voltages of voltage sources V1-V4. By repeating the above steps, the output voltages of the voltage sources V1-V4 are adjusted to approach the target values.
However, as shown in FIG. 1, each voltage source requires a corresponding pad to output the generated output voltage. Thus, the amount of pads should be equal to the amount of voltage sources, thus, taking up much chip area and requiring a lot of pins. In addition, since the external testing machine has to sequentially adjust the voltage sources, time is wasted when switching the pads. Furthermore, for some specific voltage sources that need to supply voltages with extremely precise voltage levels, such as the bandgap reference voltage generator, since the signal is weak and sensitive, it is unable to be coupled to the pads for conventional voltage adjustment.
Thus, an adjusting circuit which is capable of reducing chip area, required pins and testing time, and further providing precise adjusting results is highly required.